Fermentation production of short chain alcohols from high-starch feedstocks such as grains and tubers, generally comprises the steps of pulverizing the feedstocks, liquifaction and saccharification of starch components in the pulverized feedstocks into fermentable sugars by enzymatic hydrolysis, and then fermenting the sugars with a suitable microorganism to produce a “beer” or “mash”. The beer generally comprises alcohol, water, solubilized organic compounds produced during fermentation, and “solids” which include un-metabolized starch moieties, non-starch plant debris, and dead microbial cells. The solubilized organic compounds in fermentation beers include microbial metabolic products generally comprising organic compounds, as well as plant extractives and plant derivatives produced during enzymatic hydrolysis of the pulverized feedstocks. All of these components comprising beers are commonly referred to as fermentation co-products.
Some commercial fermentation processes are conducted in a single vessel wherein the enzymatic hydrolysis step, also referred to as saccharification, and fermentation step are combined and occur simultaneously. Such processes are commonly known as “simultaneous saccharification and fermentation” (SSF). Other commercial processes separate the saccharification and fermentation components into sequential steps that are conducted in separate vessels.
The beers produced from fermentation of high-starch feedstocks generally comprise about 5% to 7% alcohol. The alcohol product is recovered from the other products produced during fermentation, by distillation. The post-distillation residual materials represent about 90% to 95% of the fermentation products, and are commonly referred to as “whole stillage”. Whole stillage is typically separated into a solids fraction and a liquid fraction by centrifugation. The solids fraction is commonly referred to as a “wet cake” which is then further de-watered and dried to produce “Distillers Dried Grains” (DDG). DDG is used directly as animal feed and/or indirectly as feed supplements. The liquid fraction separated from whole stillage is commonly referred to as “thin stillage” and represents the largest volume output from commercial fermentation operations. Various commercial practices are used to dispose of thin stillage. In some cases, the water component of thin stillage is evaporated to produce a syrup which is then added to DDG to increase its nutrient value. In some commercial operations, thin stillage is recycled into saccharification and fermentation tanks or alternatively, into SSF tanks as a source of additional water. In this type of application, the recycled thin stillage is referred to as “backset”. Alternatively, thin stillage may simply be collected, evaporated and discharged into waste disposal systems.
Depending on the type of high-starch feedstock provided to the fermentation process and the type of fermentative microorganism(s) employed, thin stillage generally comprises a wide range of yeast metabolites, monosaccharides, di-saccharides, organic compounds released from plant materials during enzymatic hydrolysis and incompletely digested plant debris. Thin stillage components may include glycerol, ethanol, lactose, glucose, arabinitol ribitol, various non-nitrogenous and nitrogenous acids, polyhydroxyl alcohols, amino acids and proteins. Large-scale commercial fermentations generally experience some degree of bacterial contamination that usually does not negatively affect the enzymatic and biological processes. However, the presence of contaminating bacteria in the saccharification and fermentation tanks results in the significant production of bacterial metabolites such as acetic acid, lactic acid, and various organic acids. These metabolites end up in the thin stillage. At high levels, these bacterial metabolites are known to inhibit enzyme activities and microbial metabolism. Problems commonly encountered in commercial fermentations relate to the elevated concentrations of these types of bacterial metabolites in recycled thin stillage, i.e., backset, causing significant reductions in the rates of enzymatic hydrolysis in saccharification tanks, and in fermentation rates. Other problems associated with these bacterial metabolites are their known inhibitory effects on rumen bacteria, and therefore, the digestion and metabolism of DDG amended with thin stillage syrups by livestock may negatively affected.